This invention relates to an optical fiber module including a vertical cavity surface emitting laser which is coupled optically to an optical fiber with mechanical positioning.
Edge emitting semiconductor lasers are generally used in optical fiber modules at present. Since an edge emitting laser has a wide radiation angle, a lens should be inserted between the laser and an optical fiber to obtain high efficiency in optical coupling. Precise optical alignment is required for the laser, the lens, and the optical fiber in assembling the module. The alignment is usually done by operating the laser to emit a laser ray. The optical fiber is fixed at the point where output optical power from the far end of the optical fiber takes the maximum. This method causes low throughput since driving current should be applied to the laser during the assembly. Relatively long working time is required for alignment even if an automated optical-axis aligner is utilized. These factors limit the cost reduction of optical fiber modules. To avoid them, several alignment methods based on mechanical positioning have been proposed though the coupling efficiencies are usually low.
On the other hand, it has been proposed to apply vertical cavity surface emitting lasers to optical fiber modules. The performance of the surface emitting lasers has made rapid progress recently. Since the surface emitting lasers have narrow radiation angles, high efficiency can be expected even for direct optical coupling without lenses. The direct coupling of a surface emitting laser and an optical fiber is described, for example, in K. Tai et al., "Selfaligned fiber pigtailed surface emitting lasers on Si submounts," Electronics Letters, vol. 27, pp. 2030-2032 (1991) which is incorporated herein by reference. In this example, mechanical positioning of an optical fiber is attained by using a guiding hole opened in a Si submount. As shown in FIG. 4, a vertical cavity surface emitting laser 401 is formed on a light-emitting chip 402. The light-emitting chip 402 is attached to a Si submount 403. The submount 403 has a guiding hole 404 where an optical fiber 405 is inserted and fixed. The optical fiber 405 is coupled optically to a surface emitting laser 401.
The steps for aligning the surface emitting laser 401 and the optical fiber 405 are as follows. First in bumps aligned to the guiding hole 404 are formed on the submount 403. Second in bumps aligned to the surface emitting laser 401 are formed on the light-emitting chip 402. By bonding the first and the second in bumps with tension between them, the light-emitting chip 402 and the submount 403 are self-aligned. As a result, the positions of the surface emitting laser 401 and the guiding hole 404 are adjusted. Then the optical fiber 405 is inserted to the guiding hole 404 and fixed with UV cured epoxy 406, which results in the final alignment between the surface emitting laser 401 and the optical fiber 405. The surface emitting laser 401 is connected electrically to an external circuit with an n-side wire 407 bonding to the light-emitting chip 402 and with a p-side wire 408 bonding to the submount 403. By using this method, a coupling efficiency of approximately 100% is attained for a multimode optical fiber.
The method described above has an advantage that high coupling efficiency can be obtained without driving the laser during assembly. However, it is difficult to attach the submount to a package since the light-emitting chip is mounted on one side of the submount while the optical fiber is fixed to the other side. In general, the submount is attached to the package before the light-emitting chip is mounted on the submount. In this case, the floor of the package as well as the submount has a hole through which the optical fiber is inserted. In the other case where the optical fiber is taken out upward as shown in FIG. 4, the submount may be attached to the package with the light-emitting chip beneath it. A special package with a concave housing the light-emitting chip is desirably used in this configuration. In both cases, a special package with a through hole or a concave shape is required which is more expensive than a conventional package with a flat floor.